Résumé
This paper presents a reliable image and video secret storing method, aiming at ensuring the protected storage of multimedia files without risk for their owner to have their privacy violated. It relies on a particular property (standardized in MPEG-A Part.21 VIMAF) of video and image standards such as H.264/AVC, H.265/HEVC and MPEG-HEIF: a non negligible portion of the bitstream (so-called cipherable bits) can be altered without modifying the decodability of the stream. Such a modification results in an image or a video that is both a valid and visually encrypted media file. Not willing to be bothered by a complex key management system, we take inspiration from Shamir's secret sharing scheme, and propose a method where the secret to be shared is the original (unciphered) set of cipherable bits, that are encoded and divided in n shares, among which k−1 or less do not permit to reconstruct the secret, while k or more allow to recover it. Those n shares are then used to generate n ciphered fully standard compliant versions of the media. The rightful owner of the file will easily store these n versions in various Cloud storage locations, and recover them all when the fully deciphered file will be needed, while a hacker will most generally obtain only some of those files, hence not be capable to decrypt the file.